Certain processes, such as combustion of carbon containing fuels, produce gaseous emissions of carbon dioxide (CO2). CO2 has been identified as a “greenhouse” gas, which appears to contribute to global warming. Because of its status as a “greenhouse” gas, technologies have been developed to prevent large quantities of CO2 from being released into the atmosphere from the use of fossil fuels.
Chemical looping combustion (CLC) is a combustion technology that provides efficient CO2 capture and processing. CLC provides for inherent separation of CO2 produced during oxidation of carbon containing fuels thereby creating a more concentrated stream of CO2. By increasing the concentration of CO2 as part of the combustion technology, the energy and capital expenditures required to separate CO2 after combustion for capture and storage are substantially reduced.
CLC technology generally involves use of an oxygen carrier, which transfers oxygen from air to a fuel, thereby avoiding direct contact between air and the fuel. Two inter-connected reactors, typically fluidized beds, are used in the process: a fuel reactor and an air reactor. The fuel is introduced in the fuel reactor, which further receives the oxygen carrier which is typically a metal oxide. An exit flue gas stream from the fuel reactor primarily contains products from oxidation of the fuel, H2O and CO2, and reduced oxygen carriers. A stream consisting of a high concentration of CO2 may then be obtained by condensing H2O contained in the flue gas stream of the fuel reactor after reduced oxygen carriers are removed from the exit flue gas stream.
A reduced oxygen carrier formed as part of fuel oxidation reaction, is transferred to the air reactor where it is re-oxidized in the presence of air. A flue gas stream exiting the air reactor consists primarily of non-reactive components of air, such as nitrogen, oxidized oxygen carriers and unused oxygen. Oxidized oxygen carriers may be separated from the flue gas stream of the air rector for transmission to the fuel reactor. Through the use of oxygen carriers to deliver oxygen to the fuel reactor, the non-reactive components of air are expelled from the system as they exit the air reactor and are never introduced into the fuel reactor. Therefore, the products of combustion, primarily CO2 and H2O, are not diluted by non-reactive components of air in the flue gas stream of the fuel reactor.
Depending on the conditions and materials used, combustion of the fuel in the fuel reactor may be incomplete. Incomplete combustion may cause unburnts, such as hydrogen, methane, and carbon monoxide, to be present in the flue gas stream of the fuel reactor. In order to reduce or eliminate the unburnts from the flue gas stream, the unburnts are typically oxidized in a post combustion unit after combustion in the fuel reactor. Unburnts may also be present in other flue gas streams from various industrial and/or combustion applications.
One of the difficulties with oxidation of unburnts, such as in CLC systems, is that the post combustion unit requires pure or enriched oxygen gas. If air was added to the post combustion unit for oxidation, the benefits of CLC would be lost because the non-reactive constituents of air would be added to the flue gas stream of the fuel reactor prior to transmitting the flue gas stream to a gas processing unit. This requirement to provide pure or oxygen enriched gas to a post combustion units applies equally to other industrial processes and/or combustion technologies requiring post combustion oxidation in an oxygen enriched environment (e.g. an oxy-fired plant). Accordingly, post combustion oxidation requires the addition of pure or oxygen enriched gas, which is expensive both in terms of energy consumption and capital costs. Moreover, depending on the amount of unburnts requiring oxidation, combustion in pure or enriched oxygen may lead to strongly elevated temperatures, requiring cooling. Accordingly, there is a need for an improved method and apparatus for more efficient treatment of unburnts.